Fluorescence in situ hybridization (FISH) is a sensitive method for detecting small genomic alterations associated with a variety of hematological malignancies and solid tumors. FISH technology has several advantages over routine chromosomal analysis; such advantages include its ability to detect genomic abnormalities in non-viable and non-meristematic tissues, rapid turnaround times, and more Includes high resolution. However, FISH technology complements routine chromosome analysis and does not replace routine chromosome analysis for cancer diagnosis. Acute myelogenous leukemia/myelodysplastic syndromes are a group of heterogeneous blood disorders with characteristic chromosomal abnormalities that result in specific gene rearrangements. About half of AML/MDS patients may have chromosomal abnormalities, which can be classified as primary and secondary or treatment-related. Chromosomal aberrations are known to correlate with prognosis and response to therapy. Although most of these abnormalities can be detected by routine chromosome analysis, interphase FISH analysis is sensitive enough to detect low-level clones and identify genetic abnormalities of prognostic and therapeutic importance. Fluorescence in situ hybridization (FISH) is a sensitive method for detecting small genomic alterations associated with a variety of hematological malignancies and solid tumors. FISH technology has several advantages over routine chromosomal analysis; such advantages include its ability to detect genomic abnormalities in non-viable and non-meristematic tissues, rapid turnaround times, and more Includes high resolution. However, FISH technology complements routine chromosome analysis and does not replace routine chromosome analysis for cancer diagnosis. Myeloproliferative neoplasms associated with activated tyrosine kinases and with eosinophilia are a distinct group of myeloproliferative and lymphoid neoplasms. These tumors, especially tumors with rearrangements involving platelet growth factor receptor alpha (PDGFRA), platelet growth factor receptor beta (PDGFRB), fibroblast growth factor receptor 1 (FGFR1), and BCR/ABL1 fusion respond well to treatment with tyrosine kinase inhibitors. Therefore, they are called imatinib-responsive tumors. The most common chromosomal rearrangements observed in this group include t(4;12) leading to PDGFRA rearrangements, t(5;12) leading to PDGFRB rearrangements, t(9 ;22), and chromosome 8 abnormalities. Although some of these chromosomal abnormalities can be detected by routine chromosomal analysis, FISH is the preferred diagnostic method because it is more sensitive to identify and accurately characterize the underlying gene rearrangements. Fluorescence in situ hybridization (FISH) is a sensitive method for detecting small genomic alterations associated with a variety of hematological malignancies and solid tumors. FISH technology has several advantages over routine chromosomal analysis, such advantages include the ability of FISH technology to detect genomic abnormalities in non-viable and non-dividing tissues, rapid turnaround times, and more Includes high resolution. However, FISH technology complements routine chromosome analysis and does not replace routine chromosome analysis for cancer diagnosis. Acute lymphoblastic/lymphocytic leukemia (ALL) is the most common hematologic malignancy in childhood. A number of specific chromosomal abnormalities and/or gene rearrangements have been observed in patients with ALL, with an incidence of such abnormalities of 65-85% in adults and 60-70% in children. Detection of these abnormalities is important for allocating patients to different treatment groups, and it is now possible to perform chromosomal and FISH studies, especially in children, for inclusion in the Children's Oncology Group (COG) treatment protocols. Although most of these abnormalities can be detected by routine chromosome analysis, interphase FISH analysis is sensitive enough to detect low-level clones and identify genetic abnormalities of prognostic and therapeutic importance.